Controlled distribution of chemistry in ceramic systems

ABSTRACT

A method of the controlling the chemical and physical characteristics of a body formed from a powder precursor, including measuring a predetermined amount of a first particulate precursor material, wherein the first particulate precursor material defines a plurality of respective generally spherical first phase particles, adhering second phase particles to the respective first phase particles to define composite particles, forming the composite particles into a green body, and heating the green body to yield a fired body. The second phase particles adhered to the first phase particles are substantially uniformly distributed and a respective first phase particle defines a first particle diameter that is at least about 10 times larger than the second phase diameter defined by a respective second phase particle. The composite particles define a predetermined composition.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a utility patent application claimingpriority to, and based upon, co-pending U.S. Provisional PatentApplication Ser. No. 60/862,881, filed Oct. 25, 2006, and 60/974,643,filed Sep. 24, 2007.

TECHNICAL FIELD

The novel technology relates generally to the materials science, and,more particularly, to a method for controlling the distribution ofchemical components in a sintered ceramic body, cermet, metallic body orthe like.

BACKGROUND

Ceramic materials are typically prepared from powder precursors and heattreated or sintered such that the individual grains solidify into asolid body. In many cases, the desired composition includes smallamounts of a second composition added to enhance the sintering process,such as to decrease porosity or lower the temperature at which the bodymay be sintered. Small chemical additions, known as dopants, are used toenhance sintering and adjust material properties. These additions aretypically added in small amounts, less than 10% and more commonly in theone to two percent level. The distribution of these dopants is typicallyuncontrolled or ‘left to chance’, insofar as it is done in such a way asto result in a non-uniform distribution of the dopants throughout thecompact. Such a non-uniform distribution leads to defects in theresultant compact.

The current approach for introducing dopants falls into two generalcategories: salt solutions and the use of second phase particles. Saltsolutions are initially perfectly mixed, as the salt concentrationwithin the solution should be ideally uniform. As water evaporates fromthe particle compact, or even within a droplet during a spray-dryingprocess, the salt migrates with the water in the packed particlestructure. The last pocket of liquid, be it in the meniscus between twoparticles or a reservoir within an agglomerate, is where the salt willconcentrate. Once the solubility limit is reached, the saltprecipitates. If more than one dopant is used, then the precipitationprocess occurs sequentially, with the lowest solubility saltprecipitating first, etc. This can, and does, typically lead tolarge-scale segregation of the dopants.

In systems which use second phase particles, the dopant particles areoften of similar size as the primary material particles. In some cases,the dopants are often several orders of magnitude larger, leading togross segregation as the system is being milled or mixed. Large dopantparticles inherently lead to large-scale segregation and, typically, tohaving significant excess dopant in the system. Because of large-scalesegregation, additional dopant is used to compensate for segregation.This is often leads to inferior or variable material properties aftersintering. Thus, there remains a need for a process that more uniformlydistributes sintering dopants throughout the sintered body.

Another area of ceramic processing suffering form inhomogeneousdistribution of components is wash-coats. Current technology forwash-coats consists of a mixture of ceramic particles, includingtypically two or more of the following: alumina, silica (quartz andcristobalite), zirconia, and colloidal silica (amorphous silica). All ofthese particles have different surface chemistries in water, meaningthat the surface charges can be opposite on different particles and ofdifferent magnitudes. This leads to severe problems in maintainingsuspension stability over the lifetime of the slurry pots. When thesuspension becomes unstable, often indicated by excessive settling orgelation of the suspension, the pot is discarded, often meaning thatseveral tons of material are simply dumped. Having uncontrolled surfacechemistries means that the pH of the suspension shifts and thereforemust be adjusted on a daily basis.

One source of loss for investment casting systems is the failure of theshell during the cast, resulting in ceramic shell fragments in themolten metal, resulting in the rejection of the cast and significantfinancial loss. Slurry processing of the powders to create the compositegrains ensures excellent mixing of the component powders, resulting in asignificant improvement in the strength of the ceramic shell, thuspotentially reduces losses associated with shell failure. Thus, there isa need for a wash-coat system with increased stability over time.

The present novel technology addresses these needs.

SUMMARY

The present novel technology relates generally to ceramic powderprocessing, and, more particularly, to a method for controlling thedistribution of mixtures of ceramic powders at the nano scale. Oneobject of the present novel technology is to provide an improved ceramicmaterial. Related objects and advantages of the present novel technologywill be apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron micrographs illustrating uncoated alumina(left) and colloidal silica-coated alumina (right).

FIG. 2 graphically illustrates a suspension of alumina and colloidalsilica, both at pH of 3.0-4.5 (±1.0), mixed together according to afirst embodiment of the present novel technology.

FIG. 3 schematically illustrates the measured zeta-potential ofmicron-sized silica and alumina particles as a function of pH in anaqueous medium.

FIG. 4 is a schematic view of the relative sizes of the alumina andcolloidal silica (coating) particles.

FIG. 5 graphically illustrates a calculated maximum dopant particlelevel (volume %) as a function of primary matrix particle size anddopant particle size assuming spherical particles and monolayer coverageof the dopant particles on the primary matrix particles

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

For the purposes of promoting an understanding of the principles of thenovel technology, reference will now be made to the embodimentsillustrated in the drawings and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of the novel technology is thereby intended, suchalterations and further modifications in the illustrated device, andsuch further applications of the principles of the novel technology asillustrated therein being contemplated as would normally occur to oneskilled in the art to which the novel technology relates.

The present novel technology is illustrated in FIGS. 1A-6 and relates toa method for controlling the distribution of dopants in a sintered bodyby coating a comparatively large sinterable grain 10 with asubstantially smaller grain 15 of an alternative chemistry, thusproducing a composite grain 20. The larger particle 10 is referred to asthe primary matrix particle and is typically the major component of theto-be sintered ceramic or metal body. The colloidal properties of thecomposite grain 20 would be controlled by the properties of the smaller,secondary particle shell or coating 25. In systems that have more thanone dopant, a situation that is increasingly more common in the ceramicworld, either mixed-composition coatings 25 or consecutive coatings 30may be applied to the primary matrix particle 10.

Small chemical additions, known as dopants, are used to enhancesintering and adjust material properties. These additions are typicallyadded in small amounts, less than 10% and more commonly in the one totwo percent level. The distribution of these dopants is typically leftto chance, leading to clustering of the dopants and a non-uniformdistribution of the dopants throughout the sintered body or compact.

According to the present novel technology, a composite ceramic or otherrefractory material matrix grain 20 (typically 10-50 μm in diameter) isconcentrically coated with a fine-sized, typically nano-scaled, grain ofan alternative composition 15, such as an alumina grain 10 coated withcolloidal silica grains 15 (see FIG. 1). The composite grains 20 thusexhibit the surface properties of the coating 25 rather than the innercore or primary matrix particle 10, eliminating multiple surfacechemistries and promoting a substantial increase in the suspensionstability. This configuration also reduces or eliminates the commonproblem of phase separation and agglomeration of dopant particles 15.

Overview: As the present novel technology relates to the agglomeratingof small particles 15 of a first chemistry onto larger particles 10 of asecond, different chemistry, the distribution of the smaller particles15 can be much more uniform and controllable. The result is anengineered particle 20 that consists of selectively agglomerating anideally-sized, typically ceramic, matrix grain 10 (such as alumina inthis example) with a nano-scale particle 15 of another material (such asnano-scale silica particles in this example), thus creating a nano-scalecoating 25 over the matrix grain 10 to define a composite particle 20.The composite particle 20 and the method of its production exploitsdifferences in surface chemistry, either naturally occurring (as in thecase of alumina and silica) or induced through the use of pH, organicacids and/or bases, surfactants, or dispersants, etc.

As illustrated in FIG. 2, in the system 5 of alumina base particles 10coated with nanoscale silica particles 15, pH is typically used togovern the agglomeration of silica particles 15 on an alumina particle10, resulting in a coated particle 20 that now appears to look, in acolloidal sense, like the silica particles 15. The process is thatindividual suspensions 40, 45 of particulate alumina 10 and colloidalsilica 15, maintained at an appropriate pH (typically between pH 3.0 to5.0), are blended together 50, typically with agitation 55, to allow thesilica particles 15 to heterocoagulate 60 on the surface of the aluminaparticles 10 (FIGS. 1 and 6).

This heterocoagulation 60 is driven by the opposite surface chargesbetween the alumina particles 10 and the colloidal silica particles15—alumina is positively charged in this pH range, while colloidalsilica is negatively charged (see FIG. 3). After coagulation, the pH isadjusted 65, typically through the addition of a pH modifier or buffer70, such as NaOH, NH₄OH, or the like, to a pH of between about 7.0 andabout 9.0, simultaneous with vigorous agitation to prevent localizedregions of excessively high pH, to yield a stable suspension 75.Agitation can be accomplished a variety of ways. The increase in pHyields a uniformly negative charge on the colloidal silica coating 25.It is typically advisable to avoid allowing the pH to exceed pH=9.0, asthis has the potential to rapidly degrade the suspension stability andconsequently the pot lifetime.

The amount of silica 15 necessary to coat the alumina particles 10 canbe calculated based on a hexagonal close-packed array of spheres (seeFIG. 4C), although this may over-estimate the coating efficiency. It istypical that excess colloidal silica 15 be provided to ensure that allof the alumina particles 10 are coated. The presence of excess colloidalsilica 15 typically does not affect the long-term stability of thesuspension 75.

It should also be noted that the nano-particle coating 25 does notsignificantly increase the size of the alumina particle 10, asillustrated schematically in FIG. 4B (and also evident in FIG. 1B).Another approach would be to perform the heterocoagulation step followedby the addition of a cationic polyelectrolyte 80, such astetraethylamine (TEA) or a similar organic base, to impart a positivesurface charge on the composite particles 25 (with the colloidal silicasurface). The advantage of the TEA (or similar) approach is that thesystem 5 will not be sensitive to cations in solution, since the surfacecharge will be positive instead of negative (as would be imparted by thehigh-pH approach). It is similarly emphasized, however, that the pHtypically be constrained below about 9.0 to ensure long-term stabilityof the alumina particles 10, although this is less urgent in this case.

Similar charge behavior would be expected of other oxides, carbides,nitrides, or metallic particles in an aqueous medium. The opportunityfor heterocoagulation 60 is facilitated by opposite surface charges onthe particles 10, 15. In the event of similar surface charge as afunction of pH, the use of a surface active agent 80 (such aspolyelectrolytes, organic acids, or ionic surfactants) can be employedto alter the surface charge on one of the particles 10, 15 to facilitateheterocoagulation 60. Once the coating process is complete, the chargeon the composite particle 25 may then be changed, either by controllingthe suspension pH or by the addition of a surface active agent 80, tomake the composite particle 20 similarly charged to the primary particle10 to prevent unwanted agglomeration. For example, the heterocoagulationstep 60 may be followed by the addition of a cationic polyelectrolyte80, such as tetraethylamine (TEA), to impart a positive surface chargeon the composite particles 20 (as in the example of alumina particles 10coated with the colloidal silica particles 15). The advantage of the TEA(or similar) approach is that the resultant suspension 75 will not besensitive to cations in solution, since the composite particle 20surface charge will be positive instead of negative (as would beimparted by the high-pH approach).

Composite particles 25 may be extracted 85 from the suspension 75 by anyconvenient means to provide powder precursors 90 for use in forminggreen bodies 95. The green bodies are typically formed via anyconvenient powder processing technique and may then be fired 100 toyield sintered bodies 110.

Controlling Dopant Levels:

Dopant levels can be precisely controlled by the ratio of the size ofthe dopant particle 15 to that of the primary matrix particle 10.Typically, the dopant particle 15 is sized to be about ten times smallerthan the primary matrix particle 10, and a ratio of about 100:1 is evenmore typical. Through colloidal techniques, specifically the use ofstable suspensions 40, 45 controlled either by pH or by the addition ofsurface active agents 80, agglomeration of the smaller particles 15 isavoided. Furthermore, the use of stable suspensions 40, 45 promotesmono-layer coverage of the dopant particles 15 because the dopantparticles 15 that remain in suspension would have a limited drivingforce for agglomeration on a similarly-charged particle surface.

As the ratio of particle size increases, the potential for extremelysmall dopant levels becomes possible. For example, if the primary matrixparticle 10 were ten (10) microns in diameter (and assumed to begenerally spherical), and the dopant particles 15 were twenty (20) nm indiameter, it would be possible to uniformly distribute a dopant level ofapproximately 0.7% if each primary matrix particle 10 were coated. Ifthe primary matrix particles 10 were 1.0 micron in diameter for the same20 nm dopant particle 15, the calculated dopant level would be 7.2%. Theeffect of primary particle size to dopant particle size is tabulated inTable I and illustrated in FIG. 5. These calculations assume that theparticles 10, 15 are spherical and also assume a close-packed array ofsecondary articles 15 on the surface of the primary matrix particle 10.

TABLE 1 Example dopant levels in volume percent as a function of primarymatrix particle size (D) and dopant particle size (10 nm, 20 nm, or 50nm) assuming mono-disperse spherical particles. Dopant Particle Size D10 nm 20 nm 50 nm 0.200 18.15 36.30 90.75 0.500 7.23 14.45 36.13 1.003.62 7.24 18.11 2.00 1.82 3.63 9.08 5.00 0.72 1.45 3.61 10.0 0.36 0.721.81 20.0 0.18 0.36 0.91 50.0 0.07 0.14 0.36 100 0.04 0.07 0.18 200 0.020.04 0.09

If smaller dopant levels are required for a given ratio of primarymatrix particles 10 and dopant particles 15, a portion of the particles10 may be coated and the resultant composite particles 20 may then beblended with uncoated primary matrix particles 10. For example, a blendof 50% of 1.0 micron primary matrix particles 10 coated with 20 nmdopant particles 15 to yield composite particles 20 blended with 50%uncoated primary matrix particles 10 would produce a dopant level of3.6%. It is evident from these discussions that practically any dopantlevel is possible, ranging from very small (less that 0.1%) to severalpercent, and even as high as 50% (additions above approximately 15% arenot typically considered dopants, but higher addition levels are clearlypossible).

If more than one dopant is required, as is common for many ceramic andmetal materials, blends of coated primary matrix particles 20, forexample some coated with dopant “A” and some coated with dopant “B”, canbe produced in the appropriate ratio to uniformly distribute the dopantlevels of the two components. If lower dopant levels are desired, theseblended particles 10, 20 may be further blended with uncoated primarymatrix particles 10 to tailor the dopant chemistry uniquely.

This process is also applicable to the use of several dopants. That is,this technique is not limited to single dopants, or systems requiringtwo, but could be used for multiple dopants. It is also feasible toconcentrically apply the dopant coatings 25, through the appropriate useof colloidal behavior.

The present novel technology represents an excellent opportunity tocontrol the dopant concentration and distribution within ceramic andmetallic sintered particle systems. There are a number of potentialceramic systems which would be applicable. These include, among others:

-   -   i. Al₂O₃ (dopants: MgO, Y₂O₃, TiO₂, SiO₂, glasses, etc.)    -   ii. ZrO₂ (dopants: Al₂O₃, MgO, CeO, Y₂O₃, etc.)    -   iii. ZnO (dopants: Bi₂O₅, CoO, NiO, SbO₂, B₂O₃, etc.)    -   iv. Electronic ceramic materials (ferroelectrics, capacitors,        piezoelectrics, etc.)    -   v. Diamond (dopants, such as refractory metals that form        carbides): W, Mo, Ta, Nb, Ti, Co, etc.)    -   vi. SiC (dopants: C, Si, Al₂O₃, MgO, TiO₂, etc.)    -   vii. WC, cemented carbides (dopants, Co, Ni, etc.)    -   viii. Si₃N₄ (dopants: Y₂O₃, Al₂O₃, SiC, C, Si, etc.)

While the above discussion relates specifically to coatings 25 formed onsmall, generally spherical particles 10, the particles 10 may likewisebe of non-spherical shape. Further, the coatings may be formed overlarger bodies or entities 10, such as metallic catalyst or catalystsupport bodies 10 (such as for hydrocarbon cracking or exhaust gasregeneration) and the like. For example, a catalyst support body 20 maybe formed, such as from a powder precursor, by measuring a predeterminedamount of a first particulate precursor material 10 such as describedabove and forming the same into a body 10, controlling the pH of thebody 10 such that it has a first charge, and adhering a plurality ofsecond phase particles 15 to the body 10 to define a coated body 20. Thesecond phase particles 15 would have a second, opposite charge to thebody under these pH conditions, similarly as described above, and wouldthus be substantially uniformly distributed. The pH of the coated body20 may then be changed such that it would now carry a first charge.

In another embodiment, the present novel technology relates to awash-coat system 5 including an engineered grain that is of uniformsurface chemistry, eliminating the competing chemistry problem andoffering the potential for much greater stability. The pH stabilizedsuspension 75 described hereinbelow produces a composite particle 20that is negatively charged in suspension. This has the advantage ofsimplicity and ease of maintenance, but may also be relatively sensitiveto free cations in solution, such as Al³⁺, Ca²⁺, Mg²⁺, Na⁺, K⁺, NH₄ ⁺,and the like. In some systems 5 this sensitivity will not be a problem,depending on factors such as water chemistry, dissolution kinetics ofthe suspended particles, impurities introduced by the coating of thecasting molds, and the like. In systems 5 where such sensitivity posesissues, a cationic polyelectrolyte or like surfactant 80 may be added toimpart a positive charge to the shell 25, yielding a colloidal system 5that is insensitive to cations in solution. This system 5 is likewisemore sensitive to anions, such as SO₄ ²⁻, Cl⁻, NO₃, etc., but these ionsare less common and can be more easily avoided.

Additionally, the system 5 can be readily tailored for otherapplications. The description below is again focused on particulatealumina particles 10 with a colloidal silica coating 25; however, othersimilar systems are contemplated, such as for zircon particles 10 withcolloidal silica shells 25, quartz/cristobalite grains 10 with colloidalalumina coating 15, and the like. Essentially, the novel technology isapplicable to any system 5 in which the surface chemistries varysignificantly with pH. It is even possible that a dual coating 30 (onecoating 25 on top of another coating 25 on a matrix grain 10) may beemployed.

In this embodiment, as with the previously discussed embodiment, thepresent novel technology relates to a method of producing a granularceramic or metallic composite 25 by coating a comparatively large baseceramic or like grain 10 with a substantially colloidal particle 15 ofan alternative chemistry to thus produce a composite grain 20 (asillustrated schematically in FIG. 1 for alumina and silica). The surfaceproperties of the composite grain 20 are then governed by the surfaceproperties of the material 15 making up the shell 25. A specific examplewould be alumina grains 10 coated with colloidal silica grains 15. Thecomposite grains 20 thus possess the surface properties of the coatingmaterial 15 rather than the matrix particle 10, eliminating multiplesurface chemistries, and promoting substantial improvement in thesuspension stability to thus extend the lifetimes of so-produced coatingslurries 75, such as in the investment casting industry. Further, bygrading the particle size and incorporating either an intermediate-sizedor larger cristobalite particle 67 into the mixture, a wash-coat and/orstucco system 77 may be developed that promotes automatic cracking anddestruction of the coating during the cooling of a cast metal pieceformed therein by exploiting the β- to α-cristobalite inversion (whichtypically occurs at 225-250° C.).

A high-performance and suspension stable ceramic grain 20 is thusproduced for investment casting wash-coats. For example, the method maybe used to produce stucco-coats for investment casting molds. Anengineered grain 20 is produced by selectively agglomerating anideally-sized ceramic matrix grain 10, such as alumina, with anano-scale particle 15 of another material, such as silica. The resultis a nano-scale coating 25 that eliminates multiple surface chemistriesand allows the suspension properties to be maintained and stabilizedover time, thus substantially prolonging the suspension lifetimes. Inthe vernacular of the investment casting industry, this technologyextends the lifetimes of the slurry “pots”. The incorporation ofcristobalite as a third particle 67 would not undermine the suspensionstability benefits obtained from the colloidal silica coating 25 onalumina particles 10, since the surface chemistry of colloidal silicaand cristobalite are essentially identical. In other words, both arepure silica surfaces and have nearly identical reactions with water whenin suspension.

In operation, individual suspensions of alumina particles 10 andcolloidal silica particles 15 are maintained at an appropriate pH,typically between pH 3.0 to 4.5 (see FIG. 3) and are blended together50, typically with agitation 55, to allow the silica particles 15 toheterocoagulate 60 on the surface of the alumina particles 10 (see FIGS.4A-C and 6). This heterocoagulation 60 is driven by the opposite surfacecharges between the alumina and the colloidal silica—the alumina ispositively charged in this pH range, while the colloidal silica isnegatively charged. After composite particles 20 are formed throughcoagulation 60, the pH of the suspension is adjusted 65, typicallythrough the addition of such buffer chemicals 70 as NaOH, NH₄OH, or thelike, to move the pH of the solution to between about 7.0 and about 9.0.Typically, the mixture is simultaneously vigorously agitated to preventthe formation of regions of excessively high pH to yield a stablesuspension 77. The increase in pH creates a uniform negative charge onthe colloidal silica coating 25, obscuring the surface chemistry of thealumina particles 10 (FIG. 5). Typically, the mixture pH is maintainedbelow about 9.0, as higher pH will potentially reduce the suspensionstability and the pot lifetime. The amount of silica 15 typicallyrequired to coat the alumina particles 10 can be calculated assuming ahexagonal close-packed array of spheres, although this may over-estimatethe coating efficiency. Typically, excess colloidal silica particles 15are added to ensure that all of the alumina particles 10 are entirelycoated. The excess colloidal silica 15 does not significantly affect thelong-term stability of the suspension 77.

As with the previous embodiment, the heterocoagulation 60 step may beperformed first, followed by the addition of a cationic polyelectrolyte80, such as tetraethylamine (TEA), to impart a positive surface chargeon the composite particles 20 (with the colloidal silica surface shells25). The advantage of the addition of a cationic polyelectrolyte 80 isthat the system 5 will typically not be as sensitive to cations insolution, since the surface charge will be positive instead of negative(as is imparted by the high-pH approach). Again, the pH of the system 5typically does not exceed about 9.0 to ensure stability of the aluminaparticles 10.

Typically, fine-grained or coarse-grained cristobalite particles 67 areintroduced following the heterocoagulation 60 and stabilization 65 stepsand, more typically, are introduced with vigorous agitation. For largegrain size cristobalite 67, particle size typically is selected toapproximate that of the composite grain 20 (typically 14-16 μm). If afine-grained cristobalite 67 is chosen, grain size is typicallyapproximately seven times smaller than the composite grain 20, on theorder of 2.0 μm. The cristobalite grains 67 are added to exploit thevolumetric change in the grains 67 caused by the β- to α-cristobaliteinversion (typically at 225-250° C.). This displacive transformation israpid and reversible, and is sufficiently large to initiatemicro-cracking of a coating or body 190 cast or otherwise formed 185from the slurry or suspension 77, and thus facilitate the coatingremoval from a cast metal part 210. (See FIG. 6). Typically, largercristobalite grains 67 are favored for this purpose, although smallergrain sizes will also work. Additionally, fine-grained cristobaliteparticles 67 provide for better packing by filling in the voids betweenthe larger composite particles 20 and still facilitate cracking 205 of abody 190 formed therefrom upon cooling, thus assisting removal of theceramic shell 190 by cracking it to pieces 215.

For example, a stucco shell 190 may be formed by casting 185 the slurry77. The shell 190 may be filled 200 with molten metal 195 to yield acast metal body 210. The heat of the metal 195 raises the temperature ofthe cristobalite particles 67 past their transition point, and, uponcooling, the particles 67 once again change phase. The volumetricchanges accompanying the phase changes crack the shell 205, causing itto readily crumble into pieces 215 and facilitate easy removal of thecast metal body 210.

While the novel technology has been illustrated and described in detailin the drawings and foregoing description, the same is to be consideredas illustrative and not restrictive in character. It is understood thatthe embodiments have been shown and described in the foregoingspecification in satisfaction of the best mode and enablementrequirements. It is understood that one of ordinary skill in the artcould readily make a nigh-infinite number of insubstantial changes andmodifications to the above-described embodiments and that it would beimpractical to attempt to describe all such embodiment variations in thepresent specification. Accordingly, it is understood that all changesand modifications that come within the spirit of the novel technologyare desired to be protected.

1. A method of processing powders, comprising: providing a plurality offirst particles of a first composition, wherein the first particles arecharacterized by a first average size; providing a plurality of secondparticles of a second composition, wherein the second particles arecharacterized by a second average size; mixing the first and secondparticles in an environment wherein the first particles have a firstcharge and the second particles have a second, opposite charge;substantially coating the first particles with the second particles toyield a plurality of coated particles characterized as having the sameeffective surface charge sign as the second particles; wherein thesecond size is substantially smaller than the first size; wherein thesecond composition is different from the first composition; whereinthere are enough second particles to substantially coat substantiallyall of the first particles.
 2. The method of claim 1 and furthercomprising: adding a buffer to the particles to adjust the pH to bebetween about 7 and about 9; wherein the first particles are provided ina first suspension; wherein the second particles are provided in asecond suspension; wherein mixing the first and second particles isaccomplished by mixing the first and second suspensions to yield a thirdsuspension; wherein the third suspension has a pH of between about 3.0and about 5.0; and wherein the charge of the coated particles is madenegative through by adding the buffer to the third suspension.
 3. Themethod of claim 1 and further comprising; adding a cationicpolyelectrolyte to the plurality of coated particles to impart apositive surface charge to the coated particles.
 4. The method of claim1 and further comprising: forming a quantity of the coated particlesinto a green body; and heating the green body to produce a substantiallyhomogeneous body.
 5. The method of claim 1 wherein the first particlesare alumina and wherein the second particles are selected from the groupincluding silica, magnesia, yttria, titania, and combinations thereof.6. The method of claim 1 wherein the first particles are zirconia andwherein the second particles are selected from the group includingalumina, magnesia, ceria, yttria and combinations thereof.
 7. The methodof claim 1 wherein the first particles are zinc oxide and wherein thesecond particles are selected from the group including Bi₂O₅, CoO, NiO,SbO₂, boria and combinations thereof.
 8. The method of claim 1 whereinthe first particles are diamond and wherein the second particles areselected from the group including refractory metals that form carbides.9. The method of claim 8 wherein the second particles are selected fromthe group including W, Mo, Ta, Nb, Ti, Co and combinations thereof. 10.The method of claim 1 and further comprising: forming a quantity of thecoated particles into a slurry; mixing cristobalite into the slurry toyield a substantially homogeneous investment casting slurry; and castinga mold from the slurry.
 11. The method of claim 10 and furthercomprising: filling the mold with molten metal; and cooling the moltenmetal to yield a cast metal part; wherein the mold cracks apart when themolten metal is cooled.
 12. A method of controlling the dopant level ofa sintered material, comprising: preparing a first particulate precursormaterial defining a plurality of respective first phase particles;adhering second phase particles to respective first phase particles todefine composite particles; forming the composite particles into a greenbody; and firing the green body to yield a fired body; wherein thecomposite particles are characterized by an outer surface substantiallycomprised of second phase particles; wherein the composite particlesdefine a predetermined composition; and wherein the second phase issubstantially uniformly distributed throughout the green body.
 13. Themethod of claim 12 wherein the composite particles include at leastabout 93 volume percent first phase particles.
 14. The method of claim12 wherein the first particles are alumina, the second particles aresilica, and wherein before forming the composite particles into a greenbody, cristobalite is mixed with the composite particles.
 15. The methodof claim 12 wherein the green body is a mold and wherein the green bodyis heat treated by pouring molten metal thereinto.
 16. The method ofclaim 12 wherein the fired green body is a sintered body.
 17. A methodof the controlling the chemical and physical characteristics of a bodyformed from a powder precursor, comprising: measuring a predeterminedamount of a first particulate precursor material, wherein the firstparticulate precursor material defines a plurality of respectivegenerally spherical first phase particles; adhering second phaseparticles to the respective first phase particles to define compositeparticles; forming the composite particles into a green body; andheating the green body to yield a fired body; wherein the second phaseparticles adhered to the first phase particles are substantiallyuniformly distributed; wherein a respective first phase particle definesa first particle diameter that is at least about 10 times larger thanthe second phase diameter defined by a respective second phase particle;and wherein the composite particles define a predetermined composition.18. The method of claim 17 and further comprising: adding apredetermined quantity of third phase particles to the compositeparticles; wherein the third phase particles undergo a reversible phasetransformation at a predetermined temperature; and wherein thereversible phase transformation is accompanied by a volumetric shift.19. The method of claim 17 wherein the first phase particles are aluminaand wherein the second phase particles are selected from the groupincluding silica, magnesia, yttria, titania, and combinations thereof.20. The method of claim 17 wherein the first phase particles are diamondand wherein the second phase particles are selected from the groupincluding refractory metals that form carbides.
 21. A method of thecontrolling the characteristics of a body formed from a powderprecursor, comprising: measuring a predetermined amount of a firstparticulate precursor material, wherein the first particulate precursormaterial defines a plurality of respective generally spherical firstphase particles; forming the composite particles into a body;controlling the pH of the body such that it has a first charge; adheringa plurality of second phase particles to the body to define a coatedbody; wherein the second phase particles have a second, opposite charge;wherein the second phase particles adhered to the first phase particlesare substantially uniformly distributed.
 22. The method of claim 21 andfurther comprising: controlling the pH of the coated body such that ithas a first charge.